Re: Make your own linear scales

> I've always thought the "spherosyn" could be "home-shop-able"
> (precision ball bearings in a tube, read capacitively sin/cos)
>
> Also have thought about the old "wire" tape recorders...
> What about recording a sine wave (signal generator created), and
> reading it with a std. recorder head. Then looking for a way to have
> TRUE metal tape. Iron oxide?

I've thought about both of these as well, and every time I get excited
about it, I go out to the shop, grab something or other, and shake off
a whole pile of magnetized chips.

Frankly, I don't understand just what it is in the cutting or milling
process that magnetizes previously non-magnetized steel.

Anyway, the sight of all those filings clinging to things puts me off
any magnetic solution.


On the rotary front, though, I heard from guy a few years ago who told
me of his setup. I'm still not completely clear in my own mind that it
works, lacking the geometric skills to prove it one way or the other,
but the solution is more than a little intriguing.

There are basically two problems with a pulley system. The first is
slippage. While this can be reduced with a enough spring tension, it is
hard to eliminate. Also, there is some chance of wear over time as a
result of that tension.

The second is the problem of getting the pulley exactly the right size,
that pesky PI entering into the pulley diameter equation.

This fellow's solution ( and I apologize for not giving him the credit
he is due, as I cannot find the original message ) is to stretch a
length of shim stock. He used .008" stock. A pulley close to the
right size then rides along this length of shim stock and the friction
between the two turns the pulley.

The width of the stock gives a wide bearing area, and so no wear is
likely.

The pulley is tensioned against the "tape" ( shim stock ) by a pair of
bearings to either side.

-- crude ascii art --


----/.\--\*/--/.\ ---

where

--- is the shim stock
. is a bearing, about 3/4" OD, 1/4" ID, and the width of the tape.
* is the pulley

Things are aligned so that the tape rides OVER the first bearing,
slightly above the nominal centerline of the tape, UNDER the pulley
then OVER the last bearing.

The OVER distance is slight, perhaps even zero ( I'll have to think
about that as I can't remember )

The UNDER distance of the pulley is similarly small, but not zero, and
is adjustable ( as is one end of the tape, so that as the UNDER distance
is adjusted, suitable tension can be reapplied )

The pulley is machined slightly smaller than what common sense would
suggest. ( On a perfectly flat tape, then, the readings would be high.)

Here's the magic and the important point. As a result of the small
"loop" formed by the two bearings and the UNDER distance, the actual
distance the pulley travels is greater than the distance between the
ends of the tape.

By adjusting the magnitude of the UNDER distance, one can zero out the
error in the pulley diameter.

All in all, a clever solution, it seems, assuming the geometry actually
works.

Alan

--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta